Originally Posted by onesnowyowl

You don't want to use spermicide during oral sex. In fact, spermicide (while it does kill sperm) is a bad idea in general. Why? It's incredibly irritating. You don't want it anywhere near your mouth or eyes, and it can inflame vaginal tissue. You certainly don't want to accidentally ingest any.

Just get her on birth control. It's far simpler and much less mess--and the peace of mind is worth it.